This invention relates to a nickel-base alloy and more particularly to such an alloy and products made therefrom having a unique combination of corrosion resistance and age or precipitation hardenability properties in the heat treated condition and without requiring working below the alloy's recrystallization temperature.
The ever-widening search for fossil fuels has resulted in increasing demands for an alloy having improved corrosion resistance and yield strength to overcome the conditions encountered by equipment required to explore and then exploit sour wells. Particularly in deep sour wells, the conditions usually encountered are such that good pitting and crevice corrosion resistance and stress corrosion cracking resistance are required combined with high strength and ductility. In such environments Cl.sup.-, H.sub.2 S and CO.sub.2 are present at elevated pressure and temperature. The strengths required are greater than 100 ksi 0.2% yield strength (YS), preferably greater than 120 ksi, in the age hardened rather than cold worked condition because the parts do not lend themselves to being cold worked and, if at all, only with difficulty and excessive expense. An alloy capable of meeting such rigorous requirements has long been desired for use in the manufacture of components for use in sour wells. Such material would also be well suited for use in other applications involving exposure of members of complex shape or relatively large section to environments requiring outstanding resistance to chlorides and/or sulfides under high stress such as in the chemical process industry or in other industries requiring outstanding stress cracking resistance.
U.S. Pat. No. 3,160,500 granted Dec. 8, 1964 to H. L. Eiselstein and J. Gadbut relates to a matrix-stiffened alloy described as having high strength containing 55-62% Ni, 7 to 11% Mo, 3 to 4.5% Nb, 20-24% Cr, up to 8% W 0.1% Max. C, 0.5% Max. Si, 0.5% Max. Mn, 0.015% Max. B, 0.40% Max of a deoxidizer selected from the group consisting of Al and Ti and the balance essentially Fe but not more than 20%. Here and elsewhere throughout this application, percent is given as weight percent (w/o) unless otherwise indicated. The alloy is further characterized as having at least about 60 ksi 0.2% YS (414 MN/m.sup.2) at room temperature and being essentially non-age hardenable, non-age hardenable being defined in the U.S. Pat. No. 3,160,500 as a maximum increase in yield strength of 20 ksi (138 MN/m.sup.2) when subjected to a heat treatment at a temperature of about 1100 to 1300 F. as compared to the yield strength of the alloy in the annealed condition. According to the patent, the total amount of aluminum plus titanium present in the alloy is not to exceed 0.4% "as otherwise the alloys tend to become age hardenable" (Col. 2, lines 45-49). Alloys 1-3 exemplifying the claimed subject matter and two alloys (identified here as Alloys A and B) described as outside the patented invention, are set forth in Table I where the 0.2% YS (ksi) at room temperature in the annealed condition (1900 F., 1 hour) as reported in the patent are also given.
TABLE I ______________________________________ 1 2 3 A B ______________________________________ C 0.02 0.02 0.03 0.04 Mn 0.12 0.11 0.12 0.15 Si 0.05 0.04 0.11 Cr 21.68 21.41 21.44 21.76 21.4 Mo 9.10 8.83 8.99 9.07 5.1 W -- 5.32 -- -- -- Nb 4.30 4.27 4.19 4.37 1.2 Ti 0.15 0.13 0.20 0.67 Al 0.23 0.20 0.16 0.6 Ni 57.46 Bal. Bal. 50.8 Bal. Fe Bal. 1.92 3.30 Bal. 17.1 .2% YS 73.3 92 75.2 66.5 49.5 ______________________________________
With regard to Table I it is to be noted that tungsten was reported only in connection with Alloy 2. Alloy A was described as being "similar in composition" to Alloy 1 except as indicated (Pat., col. 4, lines 10 & 11). Alloy B was characterized as having "age hardened strongly but had a yield strength at room temperature of only 49,500 psi, . . . when tested after a 1900 F. anneal."
A commercial alloy has long been on sale by the assignee of this application under its trademark Pyromet 625 with the composition set forth in Table IA.
TABLE IA ______________________________________ w/o w/o ______________________________________ C 0.10 Max. Fe 5.00 Max. Mn 0.50 Max Ti 0.40 Max. Si 0.50 Max. Co 1.00 Max. P 0.015 Max. Nb (+Ta) 3.15-4.15 S 0.015 Max. Al 0.40 Max. Cr 20.0-23.0 Ni Bal. ______________________________________
Thus, while Type 625 alloy as well as other compositions of the 3,160,500 patent are characterized by outstanding corrosion resistance particularly resistance to chlorides, sulfides and carbon dioxide, combined with stability at elevated temperatures, this combination of properties was achieved by eliminating age or precipitation hardening for all practical purposes because of the prohibitively long time required at the elevated temperature required for age hardening.
U.S. Pat. No. 3,046,108 was granted to H. L. Eiselstein on Jul. 24, 1962 for an age-hardenable nickel alloy containing 0.2 Max. C, 1% Max. Mn, 0.5% Max. Si, 10-25% Cr, 2-5% or 7% Max. Mo, 3-9% Nb+Ta, 0.2-2% Ti, 0.2-2% Al, (Ti+Al.ltoreq.2.5%) 0.02% Max. B, 0.5% Max. Zr, 40% Max. Co, 40% Max. Fe and 45-80% Ni+Co with nickel.gtoreq.30% and Co.ltoreq.40%. According to the patent a preferred composition contains 0.03% C, 0.18% Mn, 0.27% Si, 21% Cr, 0.6% Al, 0.6% Ti, 4% Nb, 3% Mo, 0.009% B, 53% Ni and balance Fe. In a further variation, iron is limited to 20% Max. with 60-75% Ni+Co, Co.ltoreq.40%. While an alloy within the range of this patent has been available as Pyromet 718 (trademark of the assignee of the present application) characterized by high strength, stress rupture life and ductility at elevated temperatures, it and other compositions of the 3,046,108 patent have not provided the desired corrosion resistance in environments containing chlorides, sulfides and carbon dioxide at elevated temperatures required for use in sour wells.
European Patent Application No. 92,397 published Oct. 26, 1983, on the other hand is expressly directed to providing an alloy suitable for use in sour gas wells where corrosion resistance is required to sulfides, carbon dioxide, methane and brine (chlorides) at temperatures up to 300 C. This publication suggests that the most likely causes of failure under such conditions are sulfide stress corrosion cracking, chloride stress corrosion cracking, pitting and general corrosion. The application goes on to propose an alloy having the required corrosion resistance and high yield strength, which is cold workable but not age-hardening containing 15-30% Cr, 5-15% Mo (Cr+Mo=29-40%) 5-15% Fe (Cr+Mo+Fe.ltoreq.46%), C.ltoreq.0.06%, Al and/or Ti.ltoreq.1%, Si.ltoreq.1%, Nb.ltoreq.0.5% Mn&lt;0.3%, Bal Ni. The preferred alloy of this publication asserted to have a yield strength in excess of 1000 MN/m.sup.2 (&gt;145 ksi) is said to consist of 20-30% Cr, 7-12% Mo, (Cr+Mo=29-40% and Cr-2.times.Mo=2-12%), 5-15% Fe, Cr+Mo+Fe.ltoreq.46%, 0.05-0.5% Al and/or Ti, C.ltoreq.0.06%, Nb.ltoreq.0.5%, Si.ltoreq.0.5%, Mn.ltoreq.0.2%, Bal. Ni. Among Alloys A-X, there are six compositions outside the claimed subject matter of the 92,397 application, Alloys F-L, containing 1.9-3.1% Nb but only Alloy K contains a significant amount of Ti for consideration here. Thus, Alloy K in addition to Ni and the usual incidental elements is reported in the publication as containing 0.034% C, 24.7% Cr, 10.1% Mo, 0% Fe, 0.25% Al, 1.40% Ti and 3.1% Nb. Apart from Table I, the only reference to Alloy K to be found in the 92,397 publication is in Table IV where, in the results of chloride stress corrosion tests, Alloy K is reported to have failed in 62 days when exposed to a temperature of 288 C. in the U-bend test, the outer fiber stress of the U-bend specimen being 1310 MN/m.sup.2 (190 ksi). Alloy H containing 18.8% Cr, 7.9% Mo, 16.8% Fe, 0.007% C, 0.11% Al, 0.11% Ti, 3.1% Nb and the Bal. Ni according to Table II passed the NACE H.sub.2 S stress corrosion test with an applied stress level of 1200 MN/m.sup.2 (174 ksi) but according to Table IV, Alloy H failed the chloride stress corrosion test in 28 days. Thus, the EPA 92,397 publication leads to the conclusion that to achieve high yield strength and resistance to corrosion including stress corrosion in environments encountered in sour wells requires a non-age-hardenable alloy with no more than 0.5% columbium.
U.S. Pat. Nos. 4,400,210 and 4,400,211 granted Aug. 23, 1983 to T. Kudo et al. and Japanese Publication No. 82-203740 December 1982, are all assigned to Sumitomo Metal Ind. KK., and state they relate to alloys for making high strength well casing and tubing having improved resistance to stress corrosion cracking in media containing sulfides, chlorides and carbon dioxide such as is encountered in deep wells. The U.S. Pat. Nos. 4,400,210 and 4,400,211 (Col. 2) assert that "cold working seriously decreases resistance to stress corrosion cracking" but seek to overcome the adverse effect of cold working by the presence of Cr, Ni, Mo and W in the surface layer of a casing or tubing. These two U.S. patents and the Japanese publication specify the composition set forth therein as containing 0.5-4% of at least one of Nb, Ti, Zr, Ta, and V. The 4,400,210 and 4,400,211 patents (Col. 6) and presumably also the Japanese publication state the elements Nb, Ti, Zr, Ta and V are equivalent to each other in providing precipitation (age) hardening due to the formation of an intermetallic compound with Ni.
EPA Publication No. 82-56480 published Jul. 28, 1982 relates to a nickel base alloy having resistance to stress corrosion cracking in contact with water at elevated temperature as in boiling water nuclear reactors or pressurized water reactors. The proposed alloy is described as consisting essentially of 15-25% Cr, 1-8% Mo, 0.4-2% Al, 0.7-3% Ti, 0.7-4.5% Nb and the balance Ni, strengthened by gamma prime and/or gamma double prime. The gamma prime phase is defined as an intermetallic compound of Ni.sub.3 (Al, Ti) and the gamma double prime phase as an intermetallic compound of Ni.sub.3 Nb. This publication directly contradicts the assertions of the U.S. Pat. Nos. 4,400,210 and 4,400,211 regarding the equivalence of the elements Nb, Ti, Zr, Ta and V in providing age hardening. The EPA 82-56480 publication (page 7) states that the addition of Nb is essential for obtaining high hardenability but must be combined with at least 0.4% Al and more than 0.7% Ti to obtain an appreciable age hardenability. Of the many alloys for which specific analyses are given only one, Alloy K, a reference alloy in Table 2, contains more than 4.2% Mo. As set forth in Table 2, Alloy K contains 23.3% Cr, 8.8% Mo, 4.9% Fe, 0.04% C, 0.5% Al, 1.2% Ti, 2.4% Nb and Bal. Ni. Alloy K is noted as having cracked during forging.
There is in addition a considerable quantity of publications including patents both domestic and foreign containing broad composition ranges which overlap in varying degrees with the composition ranges set forth hereinabove but none appears to come any closer to the alloy and articles made therefrom of the present application or, more particularly, to providing a composition suitable for use in sour wells. Nevertheless, there has been an increasing need for an alloy and products made therefrom having a better combination of strength and corrosion resistance, especially an alloy and products made therefrom suitable for use in environments containing sulfides, chlorides and carbon dioxide under high stress without requiring warm or cold working. It is a significant drawback of such prior compositions as disclosed in said U.S. Pat. No. 3,160,500 and said EPA Publication No. 92,397 that substantial cold reduction is required to reach the level of strength at which parts made therefrom are intended to be used especially in the case of large or massive parts. On the other hand, age hardenable compositions as exemplified by said U.S. Pat. No. 3,046,108, though age hardenable to a desirably high strength, leave much to be desired with regard to corrosion resistance, particularly resistance to cracking under stress in media containing sulfides, chlorides and carbon dioxide as encountered in sour wells.